1. Field of the Invention
The present invention relates to data transmission systems and, in particular, to a bipolar-pattern center estimator circuit for retrieving timing information from a received signal.
2. Discussion of the Prior Art
A typical data receiver system is shown in FIG. 1. Prior to transmission, data is coded as a sequence of baseband pulses. While passing through the transmission channel, the data suffers systematic distortion and noise contamination. At the receiver end of the transmission system, the data is filtered to improve the signal-to-noise ratio, and then equalized. Timing information is retrieved from the transmitted pulse sequence by a timing extractor circuit which provides sampling time points for pulse detection and defines the data-symbol time boundaries. That is, each symbol occupies a time span defined by the inverse of the data rate.
The receiver efficiency strongly depends upon sampling point accuracy as well as upon the receiver's stability and noise immunity. Thus, the receiver timing-extractor in baseband systems must operate properly, independent of the received order of symbols, and provide periodic and reliable phase information to the receiving phase-locked-loop circuit.
In carrier modulated systems, the carrier generates periodic strobes as stable sources of timing. For example, usually the carrier is a "continuous wave" with regular, periodic zero-crossings.
In several baseband modulation techniques (e.g., return-to-zero (RZ), alternate-mark-inversion (AMI), partial-response coding), slicing at the zero level produces strobe signals indicating a signal transition. Some of these transitions may convey true information. Others are only due to noise and their presence produces a system impairment. Line magnitude and phase distortion and intersymbol-interference create additional impairments which result in false timing-signals. Thus, channel equalization is required to decrease the timing variance. However, channel equalization is usually implemented only with inherently associated noise enhancement.
Improvement in extracting timing information is obtained by selecting waveform points in time which are less sensitive or immune to distortion. For example, searching timing and specific data patterns is known as the index-free technique. However, index-free recovery produces nonperiodic zero-crossings that require costly band-pass filters to reconstruct the timing signal. Selected patterns display reduced interference at the point of phase-error observation.
Some of these techniques also suffer from the problem of decreasing phase-correction-rate because correction is performed statistically on selected patterns.
A compromise between intersymbol-interference and correction-rate obtained with some bipolar codes is to use a single mark-mark, the presence of which is highly probable, and then lock onto the zero-crossing at a center of a neighbor mark transition.